You can use 1, 2, or 4 slots at a time. If one slot is dead, that means the max is 2 slots. How did you figure put the slot was dead and not the ram? I would go ahead and RMA the board while its still under warranty. It will be worth more when you upgrade.
When your referring to a memory stick, use the word stick instead of card. When you use the word card, it makes people think your referring to a video card or sound card and so on, like something that goes in one of the PCIe/PCI slots. Just a heads up.